A planar heating device of this type is traditionally configured including: a first warming heater and a second warming heater that are connected in parallel with an electric power source; and a thermostat that is connected in series with the second warming heater and heated by an auxiliary heater connected in series with the first warming heater, as disclosed in Patent Literature 1, for example. The thermostat is set such that, once it turns off, it will not turn back on. This setting secures rapid-heating property in a warming-up stage by energizing the two warming heaters and allows continuous energization of only the first warming heater after the turning-off of the thermostat.
Moreover, a vehicular seat equipped with a seat heater generally warms a user sitting on the seat by heat generated by the seat heater, via a face cover located above the seat heater. On the other hand, however, this configuration also allows warming of a seat pad located below the seat heater; therefore, it poses a problem of a heat loss, i.e., warming something else other than the sitting user that is an object to be warmed. This results in unnecessary use of electric power.
In particular, at low ambient temperatures at which such seat heater is used, the seat pad absorbs the heat generated by the seat heater immediately after starting use of the heater. Therefore, this poses a problem of degrading rapid-heating property with which the sitting user rapidly comes to feel warmed via the face cover.
For this reason, in the conventional vehicular seat disclosed in Patent Literature 2, for example, the rapid-heating property is improved by locating the seat heater close to the face cover in such a manner that a heating element is held by wadding that located between the face cover and the seat pad. FIG. 14 shows the conventional seat with the heater disclosed in Patent Literature 2. The seat with the heater is configured including face cover 121, seat pad 122, wadding 123, and heating wire 124.
However, in the configuration of the conventional planar heating device, only the first warming heater is continued to be energized after turning-off of the thermostat. Unfortunately, the configuration offers no ability to feedback information of ambient temperatures. Therefore, a stable temperature of the warming heater becomes low at low ambient temperatures. By contrast, the stable temperature of the warming heater becomes high at high ambient temperatures. Hence, this poses a problem that the stable temperature of the warming heater varies in a manner opposite to user's desire, which impairs comfort of the user.
Moreover, after the turning-off of the thermostat, since only the first warming heater is energized with a constant electric power, the planar heating device will be held at a temperature at which a quantity of heat generated by the first warming heater is in equilibrium with a quantity of heat dissipation to the environment, resulting in almost no change in the temperature. Hence, there is a problem that the user becomes hard to feel warm, which impairs comfort of the user.
In addition, after the turning-off of the thermostat, the first warming heater is continued to be energized with the constant electric power, independently of high or low ambient temperatures, which leads to a problem of lack of energy saving.
Moreover, the conventional vehicular seat is configured to increase heat conductivity, i.e., to improve the rapid-heating property, by thinning a member located between the sitting user and the seat heater. This, however, does not improve heat insulating property toward below the seat heater. Therefore, there has been a problem that a heat loss toward below the seat heater is left unimproved, and the waste of electric power remains unimproved as well.
In addition, in the configuration of the conventional vehicular seat, the member between the sitting user and the seat heater is made thinner. This poses a problem that the configuration allows a contour of the heating wire to appear on the surface of the face cover, i.e., the contour of the heating wire appears as a concavo-convex shape of the face cover. The concavo-convex shape of the face cover causes a secondary problem that the shape, in long-term service, will permanently remain as a mark of the heating wire, and that the mark becomes more distinct with a stain of the face cover, which impairs texture and aesthetic value of the seat.    Patent Literature 1: Japanese Patent Unexamined Publication No. 2003-157955    Patent Literature 2: Japanese Patent Unexamined Publication No. 2000-342381